If you are driving your roommates car and damage it in your driveway whose insurance covers it?
Insurance follows the car. Your roommates insurance will cover the damage providing that he has "collision" coverage.
If you drive your parents car but you don't live with them and you get in an accident will their insurance cover you?
Sometimes. Sometimes insurance covers whoever is driving a particular car, and sometimes insurance covers a driver no matter whose car they are driving (as long as they have the car owner's permission). You should probably check with your insurance company to be sure, or have your parents call and ask them.